Tungsten-rhenium alloy thermocouple with compensating lead wires



A ril 28, 1970 E. E. OSOVITZ ET AL Filed Oct. 19. 1966 QS-QBZTUNGST E?AND H 2-42 RHENIUM q 74-aoZ TU NGST /v EN j AND l2 zo-zsZm-ucmumINVENTORS EUGENE E. OSOVITZ JULIUS F. SCHNEIDER BY EDWARD D. ZYSK UnitedStates Patent 3,508,975 TUNGSTEN-RHENIUM ALLOY THERMOCOUPLE WITHCOMPENSATING LEAD WIRES Eugene E. Osovitz, Englishtown, N.J., Julius F.Schneider,

Oker-Harz, Germany, and Edward D. Zysk, Livingston, N.J., assignors toEngelhard Industries, Inc., Newark, N.J., a corporation of DelawareFiled Oct. 19, 1966, Ser. No. 587,803 Int. Cl. H01v 1/14; C22c 19/00,27/00 US. Cl. l36227 5 Claims ABSTRACT OF THE DISCLOSURE The presentinvention relates to thermocouples and particularly to leg and lead wirecombinations for high temperature thermocouples.

Thermocouples are formed by connecting two wires of dissimilar metalshaving a difference in electric potential which varies as thetemperature at the junction of the wire, referred to as the hotjunction, varies. The difference in potential is measured with apotentiometer, or other instrument for measuring E.M.F. (electromotiveforce), connected across the wires at their free ends.

The thermocouple must, of course, be made of metals which withstandtemperatures in the range in which the thermocouple is intended to beused. Beyond that, metals are selected which will produce the largestand most uniform variation in E.M.F. output per degree of temperaturechange within the range in which it is desired to have the thermocouplemost accurate.

In many cases the metals which are best adapted for accurate hightemperature thermocouples are the more expensive metals. Consequently,it is desirable to be able to have the expensive thermocouple leg wiresas short as possible and use less expensive metal lead wires to connectthe legs to the instrument for measuring potential difference. Butfinding lead wires to match the leg wires is a problem.

For a match the lead wires must not distort the E.M.F. output of the legwires to such extent that it interferes with the accuracy of thethermocouple. Since the lead wires are of different metals from the legwires they form couples at the connections which produce E.M.F.responses dependent upon the temperature at the connections. Theseconnections are referred to as the leg wirelead Wire junctions and thetemperature at these junctions is determined by the heat conducted alongthe leg wires from the hot junction and by the surrounding temperature.The temperature at the leg wire-lead wire junctions is therefore lowerthan but proportional to the temperature at the hot junction and theamount of difference depends upon the length of the legs and the extentto which the leg wire-lead wire junctions are insulated from thesurroundings.

For a suitable match the E.M.F. output of a couple formed by the legwires should be substantially the same as the E.M.F. output of a coupleformed by the lead wires at the temperatures expected at the legwire-lead wire junctions when the temperature at the hot junction is inthe range in which the thermocouple is intended to Patented Apr. 28,1970 be used. In a sense the consideration of the lead wires as a coupleand the leg wires as a couple at temperatures expected at the legwire-lead wire junctions is hypothetical, since the couples which existin the operating thermocouple are formed by the junctions of therespective leg and lead wires. However, in testing for a suitable matchthese hypothetical couples are actually formed and tested within thecontemplated temperature range because if these hypothetical coupleshave similar response then the respective leg and lead combinations willhave similar responses and thus be suitably matched.

The present invention is a high temperature thermocouple having legwires of tungsten-rhenium alloys and matching lead wires. Rhenium is anexpensive metal and the present invention has the advantage of providinga thermocouple which has high E.M.F. response to temperature changeswithin a range of high temperature due to the rhenium alloyed withtungsten and yet the cost is kept down without sacrificing accuracy byproviding matching lead wires of less expensive metals.

In addition the lead wires are easier to bend and shape around cornersand obstructions than the tungsten-rhenium alloy leg wires and thusfacilitate the connection to a measuring instrument.

The objects, advantages and details of the thermocouple of the presentinvention will be more apparent from the following description andaccompanying schematic diagram of a thermocouple in accordance with thisinvention.

As seen in the diagram a thermocouple in accordance with the presentinvention has a hot junction, marked A, at which a thermoelectric legwire 11 is joined to another thermoelectric leg wire 12 of a differentmetal by welding or other appropriate means to make a good connection.The thermoelectric response of the leg wires 11 and 12 to heat at thehot junction A is measured by a potentiometer 13 or other instrument formeasuring the difference in electric potential. As mentioned above theleg wires 11 and 12 are made as short as possible and are connected tothe potentiometer 13 by lead wires 14 and 15 of less expensive metal.

As shown, the lead wires 14 and 15 are connected to the leg wires 11 and12 respectively at B and C which are the leg wire-lead wire junctions.Since the leg and lead wires are of different metals, they form coupleswhich have E.M.F. outputs determined by the temperature at the junctionsB and C. In order for the lead wires to match the leg wires the E.M.P.output of the lead and leg wire couples at the junctions B and C shouldbe closely equal over the temperature range expected at these junctions.

In accordance with the present invention the positive thermoelectric legwire is an alloy of 96-98% tungsten and 2-4% rhenium. The other ornegative leg wire is an alloy of 74-80% tungsten and 20-26% rhenium. Allpercentages of ingredients of the alloys described herein are to beunderstood as being percentages by weight.

Preferred alloys used for the leg wires are 97% tungsten and 3% rheniumfor the positive leg and tungsten and 25% rhenium for the negative legbut the lead wires described in detail below are adapted for use withleg wires whose proportions of tungsten and rhenium may be varied withinthe ranges set out in the previous paragraph.

In addition, in practice, minute traces of materials such as thoriumoxide, potassium, aluminum or silicon are sometimes added to the alloysof the leg wires to retard grain growth and thereby increase the usefullife of the wires at the high temperatures to which they are subjected.The inclusion of these materials does not alter the thermoelectricproperties of the above described leg wire alloys sutficiently to reducethe precision at which the tungsten-rhenium couples are used and it isto be understood that the above alloys with grain growth retardingmaterials added are intended to be included in the scope of theinvention.

Previous thermocouples having a 100% tungsten leg versus a leg of 74% or75% tungsten and 25% or 26% rhenium tend to fail rapidly in service dueto recrystallization and grain growth of the tungsten leg. A couplehaving a 95% tungsten-% rhenium leg versus a 75% tungsten-% rhenium legwas tested and found to provide successful high temperature operationfor substantially longer periods than a tungsten versus tungsten-25%rhenium couple. However, the 100% tungsten versus 75% tungsten-25%rhenium couple produces higher response at temperatures around 2000" C.and above.

The thermocouple in accordance with the present invention having a legof 96-98% tungsten-24% rhenium-and preferably 97% tungsten-3%rheniumversus a 75% tungsten-25% rhenium leg has advantages over coupleswith either a 100% tungsten leg or a tungsten-5% rhenium leg versus aleg of 75% tungsten-25% rhenium. The 95% tungsten-3% rhenium leg,particularly when doped with trace amounts of grain growth inhibitors asdescribed above, has a much longer useful life than the tungsten leg,and frequently in tests exhibited greater high temperature mechanicalstability than the 95% tungsten-5% rhenium leg.

The E.M.F. response of a couple of this invention having a 97%tungsten-3% rhenium leg versus a 75% tungsten-25% rhenium leg is higherthan that of a couple with a 100% tungsten leg versus a 75% tungsten-25%rhenium leg over the range tested. Its E.M.F. is notably higher than thecouple with the 95% tungsten-5% rhenium leg at all working temperaturesabove 1000 C.

Comparative E.M.F. measurements are shown in the following Table I.

TABLE 1.E.M.F. IN MlLLlVOLTS Hot junction 97% ws3 temperature W vs. 74%R0 vx.75% vs. 75% vs. 74% W- C. W26% Re IV-25% Re W-25% Re 26% ReReference junction at measuring instrument maintained at, 0 C.

The couple having a leg wire of 97% tungsten-3% rhenium versus a legwire of 75% tungsten-25% rhenium produces high and uniform variations inE.M.F. per degree of temperature change within a range from 0 to about2700" C. at the hot junction A. This couple is considered best of thecouples of the above Table 1 for use at the higher temperatures in thisrange. At the higher temperatures the couples should be protected fromoxidation. Suitable protection against oxidation is provided bysurrounding the couple with an atmosphere of hydrogen, helium, argon,and nitrogen-hydrogen mixtures. A good vacuum is also suitable.

in accordance with the present invention the negative lead wire for thenegative leg wire of 7480% tungsten- 2026% rhenium is an alloy lead wirecomposed of 05% W-5% 05% W5% Re i 9597.5% nickel, 1.5-3.0% chromium,0.53.0% aluminum, O.3-l.5% silicon and 0-1.5% tungsten. One suitablenegative lead wire used in practice is an alloy wire having 95.5%nickel, 2% chronium, 1% aluminum, 0.5% silicon and 1% tungsten. The leadwire for the positive leg wire of 96-98% tungsten-24% rhenium leg is analloy wire of 87-93% nickel and 7-13% chromium. A particular positivelead wire used in practice is composed of 90% nickel and 10% chromium.

In the usual arrangement the leg wire-lead wire junctions B and C areclose together and are the same distance from the hot junction A. Thetemperature at both junctions B and C will therefore be substantiallythe same. As previously discussed the difference between thetemperatures at the hot junction A and the leg wire-lead wire junctionsB and C is determined by the distance of the junctions B and C from thehot junction A and the extent to which the junctions B and C areinsulated from the surroundings. The range of temperatures at thejunctions B and C when the hot junction A is within the range oftemperatures which the thermocouple is adapted to measure can thereforebe adjusted and fixed by varying the length of the legs and varying theinsulation of the leg wire-lead wire junctions. It will be appreciatedthat extensive experimentation and testing are required to find leadwire combinations which closely match the leg wire combinations over anyrange of temperatures. When lead wire combinations are found whichclosely match the leg wire combination over a range of temperaturesapproximately equal in extent to the range of temperatures to which theleg wire-lead wire junctions will be subjected, then it may be desirableto shift the range effective at the leg wire-lead wire junctions byadjustment as above in order to achieve the best match.

With the hot junction A subjected to temperatures in a range up to about2700 C. which the thermocouple of this invention is intended to measure,the leg wire-lead wire junctions B and C are therefore spaced from thehot junction A and insulated so as to have a corresponding lowertemperature within the range of about 500 C. to about 800 C.

As used herein a suitable match between the leg and lead wires is one inwhich the variation in E.M.F. output of the legs per degree oftemperature change does not differ more than about 10% from thevariation in E.M.F. output of the lead wires resulting from the samechange of temperature from a temperature at which the E.M.F. outputs arethe same.

In Table II below the close match of lead and leg wires of the presentinvention are illustrated by comparing the potential difference orE.M.F. output of the leg wire couple with the potential difierence ofE.M.F. output of the lead wire couple.

TABLE ll.-E.M.F. 1N MILLIVOLTS Lead wires Leg wires 05.5% Ni, 2% Cr 07%W 3% Re vs. 1% A1, 0.5% Si Temperature C. 75% W, 25% Re and 1% WDifference B. 007 8.108 011 10.0212 10. 036 056 12. 1211 12. 085 044 14.14. 08b Oil!) It will be appreciated that the above table isillustrative of a thermocouple of the present invention having leg andlead wires made of the preferred alloys and that certain variations inthe percentages of ingredients of the alloys used may be made withoutdeparting from the spirit or scope of the invention.

Apart from the close match of the lead wires with their respective W-Rethermocouple legs, they are capable of withstanding, withoutdeterioration, temperatures from 500800 C. in an oxidizing atmospheresuch as air and they also exhibit favorable welding to their respectiveW-Re thermocouple legs.

Variations of the thermocouple and lead wire compositions arecontemplated within the scope of the appended claims.

What is claimed is:

1. A thermocouple having a leg and lead wire combination comprising apositive leg wire of 96-98% tungsten and 24% rhenium joined to anegative leg wire of 74-80% tungsten and 20-26% rhenium, a lead wireconnected to the positive leg composed of 87-93% nickel and 713%chromium and a lead wire connected to the negative leg composed of95-97.5% nickel, 1.5-3.0% chromium, 0.53.0% aluminum, 0.31.5% silicon,and 0-1.5% tungsten, said percentages of ingredients being by weight.

2. A thermocouple leg and lead wire combination as set forth in claim 1in which the positive leg wire is about 97% tungsten and about 3%rhenium.

3. A thermocouple leg and lead wire combination as set forth in claim 1in which the negative leg wire is about 75% tungsten and about 25%rhenium.

References Cited UNITED STATES PATENTS 3,296,035 l/l967 Zysk 136227ALLEN B. CURTIS, Primary Examiner M. I. ANDREWS, Assistant Examiner US.Cl. X.R.

